Both theoretical and preliminary experimental studies indicate that combined A.C. amplitude and phase Coulter type measurements are feasible. The AMAC II has been completely rebuilt into a reliable laboratory instrument with expanded dynamic range. D.C. pulses have been digitized with a Biomation transient recorder. Studies are presently in progress to determine the optimum conditions for glutaraldehyde fixation of human erythrocytes and mouse mastocytoma cells. The density gradient system controller has been rebuilt. Pulse shape studies using Fourier Analysis have indicated the possibility of cell classification by their frequency spectrum. Bibliographic references: Dunlap, L.A., Warters, R.L., and Leif, R.C., "Centrifugal Cytology III. The Utilization of Centrifugal Cytology for the Preparation of Fixed Stained Dispersions of Cells Separated by Bovine Serum Albumin Buoyant Density Centrifugation", J. Histochem. and Cytochem., 23: 369-378, (1975). Leif, R.C., Smith, S.B., Warters, R.L., Dunlap, L.A. and Leif, S.B., "Buoyant Density Separation of Cells. I. The Buoyant Density Distribution of Guinea Pig Bone Marrow Cells", J. Histochem. and Cytochem., 23:378-389, (1975).